1. Field of the Invention
This invention relates to information transceivers. In particular, this invention relates to multi-carrier information transceivers with radio frequency interference reduction.
2. Description of Related Art
Multi-carrier communications transceivers allow the high-speed transmission of information using the twisted-pair telephone lines that connect individual subscribers to a telephone central office. Each pair of copper wires provides a communication channel in which the frequency response attenuates as the frequency increases. The wires also contain noises of a different nature produced by a variety of sources. Among these noises are thermal noises produced by electric devices and cross-talk noises produced by, for example, other subscribers connected to the same central office and sharing the same bundle of twisted-pairs.
The twisting of the twisted-pairs help to reduce the cross-talk noise by limiting electromagnetic coupling between the pair of lines that are close together. However, as the frequency of operation increases, the effect of twisting is limited and the cross-talk noise increases proportional to frequency.
In order to provide reliable communications over a channel with limited bandwidth and frequency-dependent noise, multi-carrier transceivers apply a “divide and conquer” strategy. In this strategy, the total bandwidth of the communication channel is divided into a number of frequency sub-bands. Each sub-band is a sub-channel in which an information signal is transmitted. The width of the frequency sub-bands is chosen to be small enough to allow the distortion introduced by a sub-channel to be modeled by a simple complex value representing the attenuation and phase shift of the received signal. Various information signals are transmitted simultaneously using the various sub-channels. The receiver is able to separate the information signals in the different frequency sub-bands by using a bank of band-pass filters each one tuned to one of the different sub-bands. If these filters are chosen properly, the noise in each frequency band can be modeled using only the noise level present in that sub-band, with the noise in one band having little to no effect in the adjacent sub-bands.
A primary advantage of a multi-carrier transceiver is that the transceiver parameters can be optimized for different channel conditions in order to obtain maximum performance. The optimization process can be summarized as follow: First, a desired bit error rate is established. Second, the signal-to-noise ratio available in every sub-channel is measured. The bit error rate and the signal-to-noise ratio are then used to determine the maximum bit transmission rate that the sub-channel can support. Finally, an optimal set of information signals capable of transmitting this maximum bit transmission rate is found. By optimizing each sub-band, the total transmission capacity of the multi-carrier transceiver for a given error rate is maximized.
Usually, the noise in the telephone lines also contains radio frequency interference (RFI) produced by, for example, electromagnetic coupling of radio frequency signals coming from radio broadcasting transceivers that operate in the same radio frequency band as the multi-carrier transceiver. When present, this RFI can degrade the performance of the multi-carrier transceiver significantly, making the multi-carrier transceiver operate well below its optimum performance. The nature of the RFI is different from the difficulties associated with thermal noise and crosstalk noise. Optimizing a transceiver to operate in the presence of all the noises results in transceivers with great complexity, such as the transceiver disclosed by Sandberg et al. in 1995 entitled “Overlapped Discrete Multitone Modulation for High Speed Copper Wire Communications.” In practice, RFI mitigation techniques that minimize the degradation in performance are preferred.